Mumm-Ra learns about the Treasure of Thundera and the Sword of Plundarr (Jaga had beaten Ratilla, who owned the Sword, and then buried the sword deep underground), and he and Ma-Mutt ""travel back in time and space"" to explore the debris of Thundera and look for the sword, which causes the planet to reform. The Thundercats find out about this and must make the journey to New Thundera to stop Mumm-Ra. Lynx-O and the Thunderkittens are left at the tower while the other Thundercats at the Lair are preparing the Feliner for the trip to New Thundera. Vultureman notices that the tower is only guarded by 3 Thundercats and goes to Sky Tomb and tells Luna. Luna then sends Red Eye and Tug Mug to attack the Tower, and they capture Lynx-O and the Thunderkittens.

Lion-O and the other Thundercats arrive and save Lynx-o and the Thunderkittens from Red Eye and Tug Mug. The Thundercats learn more about New Thundera, especially the dangerous places affected by the moons of Plundarr where the Lunataks come from. The Thundercats must develop special equipment and clothing to counter those dangers. Also, they learn that all the snarfs have been turned into slaves and Snarf and Snarfer want to rescue them no matter what the cost. Vultureman learns of the Thundercats plan, so he abandons the other Mutants and takes the Rat-Star. But Slithe has snuck aboard. Right after the Feliner takes off, the Rat-star ambushes them and it seems all over for the Thundercats aboard the Feliner. Lion-O uses the sword to free everyone and along with Ben-Gali and Wilykit on the HoverCat, go after the Ratstar and it crashes on Hook Mt. The Thundercats leave Third Earth's atmosphere and Lynx-O tells him that Mumm-Ra has already located the Sword of Plundarr.

Mumm-Ra retrieves the Sword of Plundarr and the Thundercats arrive on New Thundera. While the Thundercats want to get the Sword and treasure, Snarf and Snarfer want to free their fellow snarfs. While the Thundercats sleep, Snarf and Snarfer sneak away, but are discovered and captured by Mumm-Ra. He disguises himself as Snarfer and he leads the Thundercats to where the treasure is located, but it can only be freed by the Sword of Omens. Once the treasure is freed, Mumm-Ra reveals himself and defeats the Thundercats with the Sword of Plundarr. Now Mumm-Ra possesses the Sword of Plundarr and the Treasure of Thundera

Mumm-Ra tries to get away but the Sword of Omens breaks open the casket dropping the treasure all over the planet, and Snarf, Snarfer, and one other snarf escape. Mumm-Ra gives a false Book of Omens to Ma-Mutt, Slithe and Vultureman, who have arrived on New Thundera in the Rat-star, for safe keeping so that he can decipher the real Book of Omens. The Thundercats go after them through the Caverns of Cold. They head toward the Canyons of Youth, but due to the effects of the canyon, Panthro, Cheetara, Tygra, Snarf, and Snarfer are changed into Thundercubs. Lion-O must use the sword to give the Thundercubs Thundercat understanding. Meanwhile on Third Earth, Jackalman, Monkian, and the Lunataks attack Cats Lair and capture Ben-Gali and Wilykit. Lion-O learns that the Thundercats back on Third Earth are being held prisoner in Sky Tomb, so he decides to send the Thundercubs to Third Earth to rescue them while he tries to get the Book of Omens from Mumm-Ra. Snarf decides to stay with Lion-O an

Lion-O frees the Thundercubs from Mumm-Ra's spell and the Thundercubs safely leave New Thundera. Lion-O and Snarf must confront Mumm-Ra and get the Book of Omens. After they defeat Mumm-Ra, they ask Mu-Mutt (using a magical flute) to use Mumm-Ra's ""device"" to get them back to Third Earth. Meanwhile the Lunataks capture some Berbils and the Thundercubs return to Third Earth and they and the Thunderkittens must go to Sky Tomb to free the other Thundercats and the Berbils. After everyone is freed, they are all cornered by the Lunataks, but Lion-O and Snarf arrive to save the day. The Thundercubs must then go in the Cave of Time to become Thundercats again and Mumm-Ra is abandoned on New Thundera and the Ancient Spirits bring him back to Third Earth.

Mumm-ra finds the Totem of Dera (a part of the T-cat treasure) which has special healing powers and can bring inanimate objects to life. He gives it to Vulture-Man and the Mutants so they can make a device to destroy the T-cats. After making the device, the mutants decide to test it near a swamp. At Cat's Lair, Lion-O sees that there is some mutant activity near the swamp. He, Wilykit, and Snarfer decide to check it out. When going to the swamp, the Hovercat is brought down by a swamp monster made by the device. The Hovercat crashes in the swamp, and Lion-O gets intoxicated by the deadly swamp water. Jaga appears and tells Snarfer that he must get the Totem of Dera to heal Lion-O. Snarfer gets the Totem and succeeds in healing Lion-O. Lion-O calls on the T-cats to try to kill the swamp monster. When all their weapons prove ineffective against the monster, the T-Cats come up with the idea to dissolve the monster in water and they succeed in defeating it.

After looking at the Book of Omens, and seeing a picture of a necklace, Lion-O discovers that there are pictures of the Thundercats on the necklace. Jaga appears to Lion-O and tells him that he is looking at a picture of the Chain of Loyalty. Jaga tells Lion-O that if the chain gets broken, it would cause the Thundercats to turn against each other. In hearing this, Lion-O and the Thundercats (Panthro, Tygra, Cheetara, and the Thunderkittens) set off to Thundera to find the chain. Mumm-ra, eavesdropping on Lion-O through his pool at the pyramid, goes to Thundera, kidnaps and disguises himself as Snarf. When the Thundercats arrive, they split up into groups to find the chain, and Mumm-Ra goes with Lion-O and the Thunderkittens. Lion-O's group finds the chain and Mumm-Ra gets it, breaks it, and returns to his true form. For a while, the Thundercats, except Lion-O, fight against each other until Lion-O uses the sword of Omens, which magically puts the necklace back together. Mumm-ra is def

Lion-O, Lynx-O, Snarf and Tygra go to the Crystal Canyon, thinking the Lunataks somehow got the Treasure of Thundera. While there, they encounter Aluro and Chilla. The two Lunataks trick the Thundercats by confusing them using with their reflections. They knock out the Thunderstrike and cause it to crash. Lynx-O is seriously injured and Lion-O is knocked unconscious. Tygra and Snarf find the Keystone and when Tygra holds the Keystone, he becomes convinced that it is making him more powerful. He uses it to restore the Thunderstrike, and heal Lion-O and Lynx-O. They return to the Lair and the other 3 Thundercats learn Tygra has become seriously addicted to the Keystone. When they try to tell Tygra he's become a slave to the Keystone, he doesn't believe them. Later in a battle against Aluro, Tygra thinks he's invincible, but Aluro proves him wrong and takes the Keystone from Tygra. Then Aluro himself becomes addicted to the Keystone. Aluro captures Lion-O and Lynx-O forcing them to come w

Vultureman creates a device that plays around with Cheetara's sixth sense. She begins to see images that aren't real, and she soon begins to lose faith in herself. Meanwhile, the Lunataks enslave Panthro with a mind control device, and tell him to make the Sky-Tomb able to fly to Thundera. When the Thundercats are looking for Panthro, Vultureman makes Cheetara see images of him being captured by Mumm-Ra, being stuck on Hook Mt., and being held at Castle Plundarr. Lion-O goes to Mumm-Ra's pyramid, and is ambushed by Luna and Amok, who capture him. Tygra goes to Plundarr and is captured by Red Eye. The Thunderkittens also get captured on Hook Mountain. Cheetara is about to go crazy, but Lynx-O tells her to listen to her heart instead of her mind. She turns into a huge ball of energy that shoots out and destroys Vultureman's device and frees Panthro and the other Thundercats.

The Thundercats capture the Lunataks and, tired of dealing with their evil ways, decide to exile the Lunataks to Exile Isle, an island in space far away from Third Earth. When the Lunataks arrive, they meet up with Captain Cracker, a pirate who the Thundercats also exiled. Together, the Lunataks and Capt. Cracker send out a distress signal to a passing ship. When the ship comes near the island, the Lunataks send up a cloud of fog, which makes the ship crash. They use the engines of the ship to push the planet out of orbit and towards Third Earth. Snarfer, on his way home from Thundera, crashes on the island when it runs into him. Lion-O goes to rescue Snarfer and stop the island, because it has a deadly gas that will blow up if it enters Third Earth's atmosphere. He goes without the Sword of Omens. He gets cornered by Capt. Cracker, but the Sword awakens and flies to Lion-O, without him even calling it. He then uses dirt to stop the engines, and the planet/island eventually returns to

While trying to figure out a way to read the Book of Omens, Lion-O tries looking through the sword at the book. He is immediately drawn into the book, where a voice asks him if he has ""the key."" Lion-O says he doesn't, and four dragon heads appear and shoot fire at him. Snarf, who saw Lion-O get sucked into the book, tries to show Cheetara what happened, and he too gets sucked in. Mumm-Ra, who also watched Lion-O get sucked in, goes to cats lair and takes the book so he can destroy it with Lion-O inside. Meanwhile, just by coincidence, Panthro finds an odd looking key on Thundera. He and Tygra head back to Third Earth with the key, not knowing what it's for. Back on Third Earth, Mumm-Ra is just about to boil the book when Lion-O calls for the sword, and it brings him and Snarf back outside. Mumm-Ra takes the sword and looks through it at the book, and he gets sucked in! But Lion-O has to save him because of the code of Thundera, and Lion-O does save him. Panthro and Tygra arrive with t

Panthro, Tygra, and Cheetara return to New Thundera in the Feliner to find more of the Treasure of Thundera, but Mumm-Ra shoots the Feliner down and, as the Fog of Despair, sends them to the Canyons of Youth, which changes them back into Thundercubs. Then Mumm-Ra disguises himself as Lion-O and tells the Thundercubs to find the treasure for him. Meanwhile, back on Third Earth, Lion-O and Snarf decide to go rescue the Thundercats on New Thundera, but they must ask the Mutants to take them to New Thundera in the Rat-Star. The Mutants agree, secretly planning to attack Lion-O once they're in space. They do attack and imprison him and Snarf on the way to Thundera, but they manage to break out. Cheetara meanwhile, using her sixth sense, locates part of the treasure, the Mirror of Truth. The real Lion-O shows up, and uses the Sword of Omens with the Mirror of Truth to restore the cubs to their proper age, and Mumm-Ra reveals himself and turns into Mumm-Ra the everliving. Cheetara uses the mi

Aluro invents a ""booster"" potion in hopes that it can make Sky Tomb able to fly to Thundera. It doesn't work, and he throws the potion into the forest. Then he notices everything in the forest has grown huge. He and Luna give the potion to the Mutants, who sprinkle it around Cats Lair. The Mutants also capture Lynx-O and Snarf. But while the Mutants are preparing to transfer Lynx-O and Snarf from their cell, a mouse in the cell who Snarf had befriended gets some of the potion and attacks the Mutants. That lets Lynx-O and Snarf analyze the potion and come up with an antidote. Meanwhile, Lion-O, Pumyra, and the Thunderkittens are fighting off huge bugs in the forest that has grown around Cats Lair. Lynx-O and Snarf arrive, sprinkle the antidote over the forest, and then watch as everything turns back to normal.

On Thundera, Mumm-Ra creates the Locket of Lies and has Mu-Mutt plant it in the Valley of the Snarfs. Eggbert and Oswald Snarf find the locket and contact the Thundercats about it. When communications break up, the Thundercats (Lion-O, Cheetara, Panthro, Tygra) go to Thundera to see the Locket. When they arrive, the Locket leads them to a place in front of an old building. But the ground they are standing on gives way and they are trapped in a well. Mumm-Ra shows up and tells them not to try to escape, but the Thundercats try anyway. The walls start closing on them, and all their efforts to stop the walls fail. Finally, Lion-O uses the Eye of Thundera and the Cat Signal to push through the wall, and they all escape

Tygra, Cheetara, and Bengali bring back a few more pieces of the Treasure of Thundera. Snarf is fascinated by a bracelet and takes it to wear around. He sees that everyone obeys his every command when he has on the bracelet. Mumm-Ra also notices this. He goes to Cats Lair, takes the bracelet, and turns Lion-O, Cheetara, Tygra, and Panthro into his slaves. He takes them to his pyramid and locks them in a cell. Snarf and the Thunderkittens decide to go rescue the Thundercats. At the pyramid, when Snarf encounters Mumm-Ra, Snarf acts like the bracelet is controlling him, but he is actually immune to the bracelet's power. Meanwhile, the Thunderkittens take back the Sword of Omens, which Mumm-Ra had taken. Mumm-Ra, now trusting Snarf, goes to sleep in his casket, and Snarf takes the bracelet back. The spell is broken and the Thundercats escape from the cell. Mumm-Ra confronts them, but Lion-O uses the Sword to lock Mumm-Ra in the cell.

Lion-O, Panthro, and Snarf set out to look for more pieces of the Treasure of Thundera, and Mumm-Ra sets a trap for them. He freezes them in the caverns of cold on Thundera. Meanwhile, back on third earth, the Luna-tacs, in need of another source of power for their ship, use exercise machines to power it up. They capture the other Thundercats and force them to workout on the machines to power up the ship. The Thundercats manage to overwork the power supply, and escape from Sky Tomb. Back in the caverns, Snarf manages to get the sword of omens to Lion-O with his tail, and they escape and return to third earth.

Back visiting their old wreckage site, Lion-O and Snarf stumble upon the Thunder-scope, a legendary thunderian telescope that locates lost thunderians, and their belongings. Mumm-Ra, interested in the Thunder-scope so he could find the treasure of thundera, ambushes Lion-O. He actually kills him in this scene. Before Lion-O dies, he manages to take out a lens from the Thunder-scope so Mumm-Ra could not completely use it. Jaga appears to Snarf, and Lion-O's dead body, and revives him. Mumm-Ra comes back for the missing piece, and Lion-O has to stop him.

Mumm-Ra discovers the power of one of the pieces of the treasure of Thundera. The Double-Headed Dragon of Doom, or the Jade Dragon possess the power to entrap people in its stomach. The legend says that Haji-Man's ancestor gave the dragon to his wife as a marriage gift, and since her father was so against their marriage, he put a curse on the dragon and it engulfed Haji-man's ancestor. Jaga freed the him, and he gave the dragon to Jaga for safe keeping. Now, the Thundercats have found the Jade Dragon, and Mumm-Ra manages to trap Lion-O and Cheetara in it. Haji-man is their only hope.

A bounty hunter known as Captain Bragg comes to Third Earth to help the Thundercats get rid of the Mutants and the Lunatacks. Capt Bragg tricks Wilykat in going with him and they first go after the Mutants. After capturing them, they go to capture the Lunataks. They capture all except Luna and Amuk, who free the other Lunataks, put Bragg in a cage and tie Wilykat to the train's engine. Lion-O and Wilykit arrive to save Wilykat and the Lunataks are defeated. All the Mutants and all the Lunataks are captured and exiled.

With the Mutants and Lunataks gone, the Ancient Spirits of Evil give Mumm-Ra until sunset to destroy all the Thundercats, and to prove they're not bluffing, they exile Ma-Mutt. Mumm-Ra spreads destruction all over Third Earth, sends Tygra, Cheetara, and Panthro to limbo, and buries Lion-O alive. Then he attacks and destroys the Tower of Omens, and the Thunderkittens barely escape, but Mumm-Ra sends them to limbo too. Snarf must go into the Book of Omens to find out how to defeat Mumm-Ra. Jaga and the Thundercats in limbo go into the Eye of Thundera to help Lion-O free himself from his rocky prison. Lion-O then battles Mumm-Ra at Cats Lair and finds out from Snarf that Mumm-Ra only has until sunset to defeat the Thundercats. Mumm-Ra fails and the Ancient Spirits of Evil banish and exile Mumm-Ra from Third Earth. With all their enemies gone, the Thundercats decide to permanently return to New Thundera